This invention relates to methods and apparatus for controlling the intensity of a fluorescent lamp. More particularly, this invention relates to methods and apparatus for providing control signals for a fluorescent lamp drive circuit to control the intensity of a fluorescent lamp. This invention also relates to fluorescent lamp circuits that include lamp intensity control circuitry, fluorescent lamp drive circuitry and a fluorescent lamp.
Fluorescent lamps increasingly are being used to provide efficient and broad-area visible light. For example, fluorescent lamps are used to back-light or side-light liquid crystal displays used in portable computer displays and flat panel liquid crystal displays. Fluorescent lamps also have been used to illuminate automobile dashboards and may be used with battery-driven, emergency-exit lighting systems.
Fluorescent lamps are useful in these and other low-voltage applications because they are more efficient, and emit light over a broader area, than incandescent lamps. Particularly in applications requiring long battery life, such as portable computers, the increased efficiency of fluorescent lamps translates into extended battery life, reduced battery weight, or both.
Liquid crystal computer displays typically are illuminated using a fluorescent lamp, such as a cold cathode fluorescent lamp (CCFL) that requires a high voltage, low current power source, and requires a much higher voltage to start than it does to maintain illumination. To insure a long lifetime, the lamp must not be operated above a maximum or below a minimum current. If a CCFL is operated at high current, the lamp becomes stressed and the lamp lifetime reduces. If a CCFL is operated at low current, the gaseous components inside the lamp will not fully ionize, and the lamp will slowly poison itself. In addition, at low currents, the lamp illumination tends to become uneven. Indeed, at low currents, the lamp may experience a so-called "thermometer effect," in which one end of the lamp is dark.
Previously known fluorescent lamp drive circuits typically provide a continuous drive signal to illuminate a CCFL. To vary the intensity of a CCFL, the magnitude of the continuous drive current may be varied. Thus, to adjust the brightness of a liquid crystal computer display that includes a CCFL, the magnitude of the continuous drive current may be reduced to dim the display, or increased to brighten the display. Because of the lamp's narrow operating current range, however, a display that uses a CCFL has a narrow dimming range.
One previously known alternative to this continuous technique uses pulse width modulation (PWM) to extend the dimming range of a fluorescent lamp. That is, rather than varying the magnitude of a continuous drive signal to the lamp, the drive circuitry provides a drive signal that switches the lamp ON and OFF from maximum current to zero current at a fixed frequency. To control the lamp intensity, the drive circuit varies the duty cycle of the drive signal. Thus, a 100% duty cycle provides maximum bulb brightness, whereas a lower duty cycle effectively dims the lamp. PWM techniques extend the dimming range of the lamp without problems associated with uneven illumination at the low end of the dimming range.
To prevent noticeable flicker or interaction with ambient lighting, the PWM frequency must be approximately 100 to 200 Hz. A problem with this PWM technique is that except when the drive circuit operates the lamp at maximum brightness, the drive circuit always switches the lamp ON at maximum current and OFF at zero current at a 100 to 200 Hz rate. Constantly switching the lamp from OFF to ON requires that the drive circuitry repeatedly supply the high voltage necessary to start the lamp, which stresses the lamp and drive circuitry, and limits lamp lifetime.
In view of the foregoing, it would therefore be desirable to provide methods and apparatus for controlling the intensity of a fluorescent lamp without reducing the lamp's lifetime.
It further would be desirable to provide methods and apparatus that combine the advantages of the continuous and PWM techniques for controlling lamp intensity.